Ideally, when light is converted to electricity in a solar cell, such as a dye sensitization solar cell, the solar cell generates electricity having a current that is proportional to the amount of incident light and a constant voltage that is dependent on the internal electric potential difference.
In practice, however, the voltage varies in proportion to the amount of incident light due to recoupling of electric charges and the like. For example, as the amount of incident light decreases, the voltage decreases as well as the current.
In order to employ a solar cell in a commercial power supply, a stable supply of electrical power is crucial. In order to supply electrical power in a stable manner, the current-voltage output from the solar cell which varies in proportion to the amount of incident light should be converted to a stable current-voltage characteristic. For example, as shown in FIG. 6, a stabilized power supply circuit 1, such as a switching power supply, is provided. The output from the solar cell main body 2 can be stabilized by providing the output from the solar cell main body 2 to the stabilized power supply circuit 1; thus, a stable electrical power can be supplied to an external apparatus.
To drive the stabilized power supply circuit 1, a certain electrical power must be supplied thereto. For this reason, the current-voltage output from the solar cell main body 2 should fall within a certain range. If sunlight is temporarily blocked, for example by clouds, the output power from the solar cell main body 2 will decrease temporarily, and the stabilized power supply circuit 1 will stop functioning since the electrical power required to drive the stabilized power supply circuit 1 is no longer supplied. As a result, a stable supply of electrical power will be no longer available.
One method to overcome this shortcoming is to install in the solar cell a secondary cell or a large-capacity capacitor which has a voltage characteristic comparable to the generation voltage of the solar cell. In this method, when a large amount of light incident on the solar cell and an electrical power with a voltage higher than a predetermined voltage is generated, the secondary cell or the capacitor is charged whereas the secondary cell or the capacitor discharges when the amount of incident light is low and an electrical power with a voltage lower than the predetermined voltage is generated. Using this method, it is possible to reduce the incidence of stopping of the stabilized power supply circuit 1.
However, addition of such a device external to a solar cell may cause an increase in power loss caused by resistance and the like, and a solar cell can only supply a power as low as a value lower than the actual power generation capacity of the solar cell. Furthermore, addition of an external device may increase the price of the solar cell since the structure of a solar cell and a power generation apparatus using a solar cell may become complex. This method is not a viable option when a low-cost photovoltaic power generation apparatus is desired.
Solar cells are used in a photovoltaic power generation apparatus in which multiple solar cell panels having a predetermined size are arranged. Although solar cell panels connected in parallel pose no problem, in a solar cell in which panels are connected in series for the purpose of reducing power loss caused by resistance or increasing an open-circuit voltage (output voltage), the output power of the apparatus as a whole will significantly decrease because not much current flows in those panels when light incident on some of the panels is blocked by clouds or obstacles.
In order to overcome this shortcoming, some countermeasures, such as independently controlling each of cells or like, are required. However, such a technique may make the whole system complex, and may decrease generation efficiency and increase price.